Intestinal fibrosis in the form of fistula and fibrostenotic strictures is a major complication of inflammatory bowel disease, particularly Crohn's disease (CD). Over 1/3 of patients with CD require surgery to relieve obstruction associated with fibrotic strictures and many require repeat interventions because of recurrent disease. Although some medications are available to target the robust inflammatory processes characteristic of CD, little is known about the mechanism of intestinal fibrosis and there are no specific therapies for this common and exceedingly morbid complication. Intestinal fibrosis is thought to follow a common pathway of fibrogenesis, as seen in other organs, in which activated fibroblasts are recruited to sites of inflammation for the purpose of wound healing. Although this fibrosis likely originates as a protective mechanism, persistent inflammation may lead to dysregulated wound healing and, in turn, excessive deposition of the extracellular matrix and fibrosis. A better understanding of the mechanism of intestinal fibrosis will likely have a dramatic impact on the treatment of CD and its complications. Although fibroblasts are considered a heterogeneous population, those that express fibroblast-specific protein 1 (FSP1) or 1-smooth muscle actin (1SMA) have been most extensively studied in the context of fibrosis. One important source of activated fibroblasts in chronic diseases of organs including the kidney, liver and heart is a process termed Epithelial to Mesenchymal Transition (EMT). EMT defines a pathway in which epithelial cells lose their phenotypic and functional characteristics, while acquiring mesenchymal features such as a spindle shape and invasive capacity. EMT is mediated by transforming growth factor-2 (TGF2) and treatment with bone morphogenic protein-7 (BMP-7) inhibits inflammation, apoptosis and EMT by counteracting the pro-fibrotic actions of TGF2. The role of EMT in intestinal fibrosis has yet to be investigated. In Aim 1 we propose to use transgenic mice in which intestinal epithelial cells are lineage tagged to characterize activated fibroblasts originating from EMT in the setting of TNBS-induced Crohn's colitis. In Aim 2 we will first evaluate whether EMT occurs by a TGF2-induced Smad-dependent pathway in vitro and then address the role of TGF2 receptor signaling in vivo by inducing TNBS Crohn's colitis in mice that lack a functional TGF2RII in intestinal epithelial cells. We will also evaluate the role of Smad- mediated TGF2-signaling in TNBS Crohn's colitis by experimenting in mice that lack Smad4 in intestinal epithelial cells. In Aim 3 we will address the effect of BMP-7 treatment on EMT in vitro and intestinal fibrosis in TNBS Crohn's colitis. We will utilize the transgenics lacking Smad4 in intestinal epithelial cells to determine whether BMP-7 treatment exerts its actions purely through Smad-dependent inhibition of pro-fibrotic TGF2 signals or via alternate pathways. Finally, we will assess the synergistic properties of BMP-7 and anti-TNF1 therapy on intestinal fibrosis in TNBS Crohn's colitis. Collectively, our studies will offer new mechanistic insights into the emergence of intestinal fibrosis as well as novel targets for therapeutic interventions. PUBLIC HEALTH RELEVANCE: Intestinal fibrosis is a major and clinically significant complication of idiopathic inflammatory bowel disease (IBD), which includes Crohn's disease (CD) and ulcerative colitis (UC). While fibrosis is more commonly associated with CD, it is also sometimes seen in chronic UC. The mechanisms by which intestinal fibrosis occurs in the setting of IBD are incompletely understood and, in the absence of a defined mechanism, it is difficult to explore specific therapies to halt or reverse this process. Here, we propose to examine the underlying mechanism of fibrosis in the context of IBD by evaluating the contribution of Epithelial to Mesenchymal Transition (EMT) to intestinal fibrosis associated with a mouse model of CD. Using transgenic mouse models with specific molecular disruption of pathways involved in fibroblast proliferation/activation and EMT, we will attempt to understand the genetic and biochemical underpinnings of this process in intestinal fibrosis. We will also explore the potential therapeutic efficacy of Bone Morphogenic Protein-7 (BMP-7) as a means to boost this antifibrotic pathway.